1. Field of the Invention
The Present invention relates to a surface mounting-type piezoelectric vibrator.
2. Background Information
A piezoelectric vibrator in which a vibrating portion and a frame are formed so as to be integral with each other is held at a frame portion between covers as shown in FIG. 6 has heretofore been known. In a piezoelectric vibrator of such a construction, covers 10, 20 have a recessed shape, and an inner surface (bottom surface of each of recessed portions) of the covers is generally made of a single flat surface (Refer to JP-A-5-199056).
Since each of the recessed portions on inner side of the covers in a related structure is thus made of a single flat surface, a free end 35 of a vibrating portion 30 impinges in some cases as shown in FIG. 6 upon one of inner surfaces 15, 25 of the covers 10, 20 when the vibrator falls to cause the vibrating portion to be bent due to an impact imparted at the mentioned time to the vibrating plate. When a specially strong impact is imparted to the vibrating plate, the free end impinges upon the inner surface of a cover strongly, so that the free end of the vibrating portion chips off. This causes a frequency constituting one of the important characteristics of the vibrator to be varied greatly in some cases. Incidentally, in a tuning fork quartz oscillator for timepieces, the standards of frequency deviation are about 10 to 20 ppm. On the other hand, when a chip occurs on a free end of a vibrating portion of the oscillator as mentioned previously, the frequency varies by not lower than several hundred ppm in some cases, this posing a big problem.
In addition, in order to solve such a problem as mentioned above, a deep and large space is provided on an inner side of the inner surfaces of the covers so that these inner surfaces are separated from the vibrating portion for the purpose of substantially preventing the free end of the vibrating portion from impinging upon the inner surface of a cover. Therefore, the thickness of the covers tends to increase, so that it becomes difficult to deal with a problem of the reduction, which is now strongly demanded, of the thickness of the vibrator.
In view of these circumstances, the present invention has been made so that a structure of a piezoelectric vibrator capable of preventing the occurrence of breakage ascribed to an impact imparted to a vibrating portion is attained without increasing a total thickness of the vibrator. To provide such a structure, the invention has covers, which are used as a case for the piezoelectric vibrator, provided with inner surfaces having at least two-stage recesses. In this structure, the collision of a free end of a piezoelectric vibrator with an inner surface of the case can be prevented by increasing the depth of the two-stage recesses from the side of a base portion of a piezoelectric vibrating plate toward that of the free end section thereof, and by providing the portions of the inner surface of the case which is on the upper and lower sides with respect to the direction of the thickness of the free end section of the vibrating portion with deeper recesses extending from the part of the inner surface of the case which corresponds to a lengthwise intermediate section of the vibrating portion of the piezoelectric vibrating plate to the part of the same inner surface which is beyond the free end of the vibrating portion.
The depth of first recesses constituting shallower portions of the two-stage recesses accounts for 2% to 25% of the thickness of the covers, and the depth of second recesses constituting deeper portions of the two-stage recesses accounts for 25% to 60% of the thickness of the covers. A difference in depth occurring on boundary sections between the first recesses and second recesses is set so that stepped sections of the recesses receive an intermediate section of the vibrating portion which is bent due to an impact imparted to the vibrator. When the stepped sections are formed, in the case of a vibrating plate fixed at one end thereof, in positions away from the fixed sections thereof by a distance corresponding to 40% to 80% of a length of the vibrating plate measured from the fixed section thereof to the free end thereof, excellent results are obtained.
According to such a method, the intermediate section of the vibrating portion impinges upon the depth-different sections between the first recess and second recess even when the vibrator falls to cause the vibrating portion to be bent due to an impact imparted to the piezoelectric vibrating plate, so that a further bend of the vibrating portion is prevented. Therefore, the free end of the vibrating portion does not impinge upon the inner surfaces of the covers, i.e., the chipping of the free end of the vibrating portion can be prevented.